1. Field of the Invention
This invention relates to an interconnection structure for semiconductor integrated circuits (LSI) and the manufacture thereof.
2. Description of the Related Art
At present, interconnection used for semiconductor integrated circuits include Al and Al alloys wherein Si or Cu is added to Al. Since Al is employed as a main component for the interconnection, the allowable current density is limited to the range not higher than (2 to 3).times.10.sup.5 A/cm.sup.2. When a current exceeding the above-indicated allowable current density is applied to the interconnection, it will be broken down owing to the electromigration. Especially, in recent years, there is a growing tendency toward the reduction in size of the interconnection accompanied by an increase in degree of the integration. Accordingly, the problem on the electromigration becomes more severe.
Aluminum alloys generally have a high electric resistivity. As an interconnection is made narrower, the resistance thereof inevitably increases. This results in the great increase of the interconnection delay time owing to the large time constant. Accordingly, the merit of improving the switching velocity of the transistor will be offset. Another problem arises in that reliability becomes low owing to the heat generation due to the high density current applied to such an interconnection with a high resistance.
Under these circumstances, in order to improve the electromigration resistance of the interconnection and to lower the resistance thereof, there has been proposed the use of a Cu interconnection, which is substantially made of Cu having a high electromigration resistance and a low resistivity, in place of the Al or Al alloy interconnection.
However, Cu is more easy to diffuse into Si (substrate) or SiO.sub.2 (insulating film) than Al. When Cu is used as an interconnection material, there arises the problem that Cu diffuses to the active region thereby forming an acceptor level. This causes the carrier density to be reduced, thereby degrades the characteristics of the transistor. To solve this problem, there have been proposed techniques wherein Cu is prevented from diffusing by forming a barrier film made of various types of materials as an underlying layer or a covering layer for the Cu wire (such as set out, for example, in Japanese Laid-open Patent Application Nos. 53-116089, 63-73645, 63-156341 and 1-204449). However, the diffusion of Cu through the barrier film into Si or SiO.sub.2 cannot be well prevented, with satisfactory results being not obtained. Further, there have been proposed a technique of forming a barrier film which is made of a nitride such as Ti--N, W--N or the like or a carbide such as Ti--C, W--C or the like (U.S. Pat. No. 4,985,750) and a technique wherein a barrier film is formed by oxidizing the surface of Ti--N film (U.S. Pat. No. 5,236,869). However, the nitrides have a high electric resistivity, thus presenting the problem that the resistance of the interconnection with nitride barrier layer becomes higher than that of the interconnection with the barrier film made, for example, of W.
Moreover, there has been proposed the use of nitrides and borides of such as of Zr, Ti, Ta and the like as an underlying layer for the Cu wire (Japanese Laid-open Patent Application No. 1-202841). Although adhesion to an insulating film is improved, Cu cannot be adequately prevented from diffusing.
Like Cu, Au and Ag are lower in electric resistivity than Al and exhibit a high electromigration resistance, thus being promising. However, they are easy to diffuse into Si or SiO.sub.2 as Cu. This requires a barrier film which has a high barrier effect and does not increase the resistance of the interconnection electrode structure.
With the Al electrode, it is usual to use an alloy having Cu added thereto so as to improve the electromigration resistance of the interconnection. In this case, a barrier film is necessary for preventing the Cu from diffusing.